If your product works in the free Kontakt Player, then it will include a serial number for registering with Native Access. You can find it at the top of your download page, accessible from the confirmation e-mail you received when you purchased your library. If your download page includes more than one download link, your license will be located under the last one at the bottom of the list, as shown below:
NOTE: If you purchased the cheaper "Full Kontakt" version of any of the RealiVox product series (Ladies, Teresa, Blue), then there is no serial number necessary.
This means there's no need to "Add Library" inside Kontakt or Native Access. To load, you simply click the "Files" button at the top of the Kontakt interface, or the "Files" tab in the sidebar browser, or using Quickload. And You're all set!